Zoom lens

ABSTRACT

A zoom lens includes at least two lens groups, i.e., a first lens group and a second lens group. The second lens group includes a first sub-group near to the first lens group and a second sub-group far from the first lens group, and the first lens group and the first sub-group in the second lens group together form a substantially afocal system in at least a portion of a zooming range. In the zoom lens, focusing is effected by the first sub-group in the second lens group being moved along the optical axis. Considering a case where this zoom lens is applied for auto focusing, it is preferable that the first lens group be disposed on that side in the lens group which is most adjacent to the object side.

This is a division of application Ser. No. 07/995,414 filed Dec. 18,1992 pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a zoom lens.

2. Related Background Art

Generally in a zoom lens, focusing is effected by that lens groupthereof which is most adjacent to the object side being moved. Thisfocusing method is still used in many zoom lenses because in a zoomingrange for objects on the same distance, focusing can be accomplished bya substantially constant amount of movement and moreover the structureof the lens barrel can be constructed relatively simply.

However, that lens group of a popular zoom lens which is most adjacentto the object side requires the largest lens diameter and therefore, thezoom lens becomes heavy, and this leads to the disadvantage that in caseof auto focusing, quick focusing becomes difficult.

So, in recent years, the lighter weight of a lens group for focusing hasbeen desired with the spread of auto focus cameras and various focusingsystems have been proposed.

Zoom lenses disclosed, for example, in Japanese Laid-Open PatentApplication No. 52-109952, Japanese Laid-Open Patent Application No.56-21112, Japanese Laid-Open Patent Application No. 57-4018 and JapaneseLaid-Open Patent Application No. 1-204013 are known.

In the zoom lens disclosed in the above-mentioned Japanese Laid-OpenPatent Application No. 52-109952, focusing is effected by a part of thatlens group thereof which is most adjacent to the object side beingmoved, and in a zooming range for objects on the same distance, focusingcan be accomplished by a substantially constant amount of movement andthe structure of the lens barrel can be constructed relatively simply.However, the full length of the lens group which is most adjacent to theobject side is increased and a movable lens for focusing is disposed inthe lens group of large lens diameter which is most adjacent to theobject side, and this leads to the disadvantage that a great decrease inthe weight of the focusing group cannot be very much expected. In thezoom lens of Japanese Laid-Open Patent Application No. 56-21112,focusing is effected by some of lens groups posterior of the object sidelens group being moved, and in a zooming range for objects on the samedistance, focusing can be accomplished by a substantially constantamount of movement and the structure of the lens barrel can be maderelatively simple. However, this zoom lens is restricted to thefour-group afocal zoom lens type and is not popular.

Also, in the focusing system disclosed in Japanese Laid-Open PatentApplication No. 57-4018, any other lens group than that lens group whichis most adjacent to the object side is used as a focusing group.Therefore, the amount of movement differs depending on each vari-focusedstate for objects on the same distance. Specifically, focusing iseffected by the use of a zooming cam and a focusing cam which arefunction-approximated to each other in the correction of the amount ofmovement. This method has the feature that a group small in thefluctuation of a short distance or a group small in weight can be freelyselected, but has the disadvantage that the structure of the lens barrelbecomes complicated.

A system for an integral lens type camera is disclosed in JapaneseLaid-Open Patent Application No. 1-204013. This is a system in whicheach vari-focused state is read by an electrical signal and an amount ofmovement conforming thereto is found. This system also has the featurethat a group small in the fluctuation of a short distance or a groupsmall in weight can be freely selected, but has the disadvantage that itcannot be used as a zoom lens for a single-lens reflex camera of themanual focusing type.

Further, in a lens exclusively for auto focusing or the like, when thecorrection of the amount of movement is effected in accordance with thefocal length within the zooming range, it is preferable that thefollowing condition be satisfied in the entire zooming range:

    |β.sub.F |>1.8                      CONDITION (2)

The above-mentioned substantially afocal system refers to a lens systemin a state in which the lateral magnification β_(F) of a first sub-groupin a second lens group in case of an infinity state is |β_(F) |≧100.

When optical systems approximate to each other in small thickness aresupposed, in succession from the object side, a lens group on the objectside which is not concerned in focusing is defined as (A), a focusinglens group concerned in focusing is defined as (F) and on the imageside, a lens group on the image side which is not concerned in focusingis defined as (B). The focal length of the lens group (A) on the objectside which is not concerned in focusing is defined as f_(A), the focallength of the focusing lens group (F) is defined as f_(F), the lateralmagnification of the focusing lens group (F) when the photographingdistance is infinity is defined as β_(F), the photographing distance(the distance between object images) is defined as R, and the amount ofmovement by which the focusing lens group (F) is moved to be focused onan object at the photographing distance R is defined as Δx (the movementfrom the object side to the image side is positive). Also, the length ofthe entire lens system is defined as TL (the length from the object sideprincipal point to the image plane of the lens group (A) on the objectside).

When the distance from the object side principal point of the lens group(A) on the object side to the object is D₀, the amount of movement Δxfor focusing is expressed by the following equation I as described in apublication Optics, Vol. 12, No. 5, October 1983, pp. 359-366 ("The NewFocusing System for Zoom Lens"). ##EQU1##

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to eliminate thedisadvantages peculiar to the focusing systems in the above-describedzoom lenses according to the prior art and to enable a new focusingsystem which effects focusing by lens units other than the lens unitwhich is most adjacent to the object side being moved and yet which islight in weight and advantageous for auto focusing and can further copewith manual focusing to be realized by simple lens barrel structure.

A zoom lens according to the present invention includes at least twolens groups, i.e., a first lens group and a second lens group. Thesecond lens group includes a first sub-group near to the first lensgroup and a second sub-group far from the first lens group, and thefirst lens group and the first sub-group in the second lens grouptogether form a substantially afocal system which will be describedlater in detail in at least a portion of the zooming range. In this zoomlens, focusing is effected by the first sub-group in the second lensgroup being moved along the optical axis.

Considering a case where this zoom lens is applied for auto focusing, itis preferable that the first lens group be disposed on that side in thelens group which is most adjacent to the object side.

Further, to effect more effective focusing, it is preferable that thefollowing condition be satisfied in the entire zooming range:

    |β.sub.F |>3.3,                     CONDITION (1)

where |β_(F) | is the absolute value of the lateral magnification β_(F)of the first sub-group in the second lens group in an infinity state.

Further, equation (I) can be approximately rewritten into the followingequation (II): ##EQU2##

When β_(F) is infinity or sufficiently great, equation (I) and equation(II) can be approximately rewritten into the following equation (III):

    (D.sub.0 -f.sub.A)Δx=f.sub.A.sup.2                   EQUATION (III)

A case where as in the present invention, the second lens group isdivided into first and second sub-groups and in the vari-focused stateof at least a portion in the zooming range from the wide angle end tothe telephoto end, the first lens group and the first sub-group in thesecond lens group together form a substantially afocal system and thefirst sub-group in the second lens group is moved along the optical axiscorresponds to a case where the lateral magnification β_(F) of the firstsub-group in the above-mentioned equation (I) or (II) is infinity orsufficiently great. Thus, in such case, equation (III) is established.

In the present invention, the focal length f_(A) of the lens group (A)on the object side is equal to an invariable value f₁ duringvari-focusing, and equation (III) becomes

    (D.sub.0 -f.sub.1)Δx=f.sub.1.sup.2 ≅constant.EQUATION (IV)

Next, considering that the variation in the distance D₀ from the objectside principal point of the lens group (A) on the object side to theobject by vari-focusing at the same photographing distance is relativelysmall, equation (IV) can be approximated as follows:

    Δx=f.sub.1.sup.2 /(D.sub.0 -f.sub.1)≅constant.EQUATION (V)

Thus, when the lateral magnification β_(F) is sufficiently great in allthe zooming range from the wide angle end to the telephoto end, equation(V) is established in all the zooming range.

The amount of movement of the first sub-group in the second lens groupcan be made substantially constant in the entire zooming range.Therefore, even if the amount of movement is made constant, the amountof movement of the image plane in vari-focusing will pose no problem inpractical use, and the hardware structure of the first lens group of thepresent invention can be made as simple as the hardware structure of themovement of the lens group on the object side in the prior-art zoomlens.

Also, when the zoom lens of the present invention is used for autofocusing or the like, if the first lens group is disposed most adjacentto the object side, the weight of the focusing group can be made verylight because the effective diameter of the second lens group is verysmall as compared with that of the first lens group and moreover thesecond lens group is divided into the first sub-group for focusing andthe second sub-group for non-focusing, and this is very advantageous forfocusing by auto focusing.

When manual focusing is considered, it is desirable that theaforementioned CONDITION (1) be satisfied in the entire zooming range.

If the range of CONDITION (1) is exceeded, the amount of movement of theimage plane in the vari-focusing when the amount of movement is madeconstant will become too great and it will become necessary to effectcam correction or the like to thereby vary the amount of movement. Also,in case of a zoom lens for a manual focusing type single lens reflexcamera, the structure of the hardware becomes complicated, and this isnot preferable.

However, if the correction of the amount of movement is possible even ifCONDITION (1) is exceeded, the focusing group can be made very light inweight, and this is suitable as a focusing system for auto focusing. So,when the correction of the amount of movement is taken into account, itis desirable that the aforementioned CONDITION (2) be satisfied.

If the range of CONDITION (2) is exceeded, it will be possible to varythe amount of movement by cam correction or the like, but the lensspacing necessary for focusing will further widen and the full length ofthe lens will become great, and this is not preferable.

Further, it is desirable that the following condition be satisfied whenthe focusing according to the present invention is effected:

    4<f.sub.1 (f.sub.1 +d.sub.F,R)/(d.sub.F,R ×f.sub.T)<15,(3)

where

d_(F),R : the space between the first sub-group and the second sub-groupin the second lens group during the infinity state;

f₁ : the focal length of the first lens group;

f_(T) : the focal length of the entire system at the telephoto end.

Conditional expression (3) is concerned with the focal length of thefirst lens group and the space between the first sub-group and thesecond sub-group in the second lens group during the infinity state.

If the upper limit of conditional expression (3) is exceeded, the spacenecessary for focusing will be deficient and therefore, the close-upphotographing distance will become long, and this is not preferable.Even if the space is secured, the refractive power of the first lensgroup will become weak and it will become difficult to assume a goodzoom disposition and it will also become difficult to well balance thefluctuations of various aberrations during vari-focusing. If conversely,the lower limit of conditional expression (3) is exceeded, the spacenecessary for focusing can be sufficiently secured, but the full lengthwill become great, and this is not preferable. If the increase in thefull length is suppressed, the refractive power of the first lens groupwill become strong and it will become difficult to assume a preferablezoom disposition and it will also become difficult to well balance thefluctuations of various aberrations during vari-focusing.

Embodiments of the present invention will hereinafter be described withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the lens construction of a first embodiment of the zoomlens of the present invention.

FIG. 2 shows the lens construction of a second embodiment of the zoomlens of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the lens construction of a first embodiment.

The zoom lens of the first embodiment comprises, in succession from theobject side, a first lens group G1 comprising a negative meniscus lenshaving its convex surface facing the object side, a biconcave lens, abiconvex lens and a positive meniscus lens having its convex surfacefacing the object side, and having negative refractive power as a whole,a second lens group G2 comprising a front group (hereinafter referred toas the focusing group G2F) comprising a cemented lens, and a rear groupG2R comprising a positive meniscus lens having its convex surface facingthe object side, and having positive refractive power as a whole, a stopS, a third lens group G3 comprising a cemented lens, and a fourth lensgroup G4 comprising a positive meniscus lens having its convex surfacefacing the object side, a negative meniscus lens having its convexsurface facing the object side, a biconvex lens and a cemented lens.

FIG. 2 shows the lens construction of a second embodiment.

The zoom lens of the second embodiment comprises, in succession from theobject side, a first lens group G1 comprising a cemented lens and apositive meniscus lens having its convex surface facing the object side,and having positive refractive power as a whole, a second lens group G2comprising a front group (hereinafter referred to as the focusing groupG2F) comprising a cemented lens, and a rear group G2R comprising acemented lens and a biconcave lens, and having negative refractive poweras a whole, a third lens group G3 comprising a positive meniscus lenshaving its concave surface facing the object side and a cemented lens,and a fourth lens group G4 comprising a positive meniscus lens havingits convex surface facing the object side, a positive meniscus lenshaving its convex surface facing the object side, a negative meniscuslens having its convex surface facing the object side, a biconvex lensand a negative meniscus lens having its concave surface facing theobject side. A stop S is disposed between the negative meniscus lenshaving its convex surface facing the object side and the biconvex lensin the fourth lens group G4.

From each embodiment, it is seen that the arrangement of the refractivepowers of the first lens group G1 and the second lens group G2 may benegative and positive, or positive and negative.

The numerical data of the respective embodiments of the presentinvention will be shown in Tables 1 and 2 below. In these tables, thenumbers at the left end represent the order from the object side, rrepresents the radius of curvature of each lens surface, d representsthe spacing between adjacent lens surfaces, the refractive index n andAbbe number ν are values for d-line (λ=587.6 nm). FN represents Fnumber, 2ω represents the angle of view, d₀ represents the distance fromthe object to the lens surface which is most adjacent to the objectside, Bf represents the back focal length, and β is a value for thephotographing magnification. Also, the shape of the aspherical surfaceis expressed by the following equation: ##EQU3## where h: the heightfrom the optical axis;

X(h): the distance in the direction of the optical axis to thetangential plane of the aspherical surface at the height h from theoptical axis;

r: the paraxial radius of curvature;

k: cone constant;

C_(2i) : the 2ith order aspherical surface coefficient.

                  TABLE 1                                                         ______________________________________                                        (Numerical Data of First Embodiment)                                          f = 20.5-34.0,  FN = 2.89,  2ω = 95.2°-64.6°                      r        d           υ                                                                           n                                          ______________________________________                                        1       49.529   2.50        49.4  1.77279                                    2       19.409   13.00                                                        3       -82.454  2.00        47.5  1.78797                                    4       38.992   2.80                                                         5       153.969  4.00        31.6  1.75692                                    6       -153.969 0.20                                                         7       34.815   3.50        31.6  1.75692                                    8       51.692   (d8 = variable)                                              9       43.020   1.20        29.5  1.71736                                    10      22.359   5.50        69.9  1.51860                                    11      -56.755  (d11 = variable)                                             12      48.361   3.00        53.9  1.71300                                    13      436.187  (d13 = variable)                                             14      -63.760  1.20        52.3  1.74810                                    15      22.250   4.00        25.4  1.80518                                    16      76.693   (d16 = variable)                                             17      28.759   3.00        50.8  1.65844                                    18      68.431   1.50                                                         19      67.908   2.00        26.1  1.78470                                    20      27.642   2.50                                                         21      2058.323 4.00        45.4  1.79668                                    22      -50.144  0.20                                                         23      105.997  8.00        57.0  1.62280                                    24      -20.350  1.70        23.0  1.86074                                    25      -44.631  (Bf = variable)                                              ______________________________________                                        (variable spacing for vari-focusing)                                          ______________________________________                                        f       20.5000       28.0000  34.0000                                        d.sub.0 ∞       ∞  ∞                                        d.sub.8 17.8289       6.3818   1.5051                                         d.sub.11                                                                              3.3045        3.3045   3.3045                                         d.sub.13                                                                              3.5982        8.1846   11.7305                                        d.sub.16                                                                              9.1470        4.5606   1.0148                                         Bf      38.5995       45.1515  50.2170                                        β  -0.0518       -0.0703  -0.0857                                        d.sub.0 361.7220      366.6170 366.4282                                       d.sub.8 19.8364       8.3602   3.5272                                         d.sub.11                                                                              1.2969        1.3261   1.2824                                         d.sub.13                                                                              3.5982        8.1846   11.7305                                        d.sub.16                                                                              9.1470        4.5606   1.0148                                         Bf      38.5995       45.1515  50.2170                                        ______________________________________                                        (the aspherical surface coefficient of the                                    first surface)                                                                ______________________________________                                        cone constant: k = 0.1000 × 10                                          aspherical surface constant:                                                                       C.sub.2 = 0.0000                                                              C.sub.4 = 0.4780 × 10.sup.-5                                            C.sub.6 = 0.4468 × 10.sup.-8                                            C.sub.8 = 0.7609 × 10.sup.-11                                           C.sub.10 = 0.1215 × 10.sup.-13                     ______________________________________                                        (condition-corresponding numerical values)                                    ______________________________________                                        (1)           |β.sub.F |.sub.Min = 5.858               (3)           f.sub.1 (f.sub.1 + d.sub.F,R)/(d.sub.F,R × f.sub.T) =                   7.796                                                           ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        (Numerical Data of Second Embodiment)                                         ______________________________________                                        f = 82.0-196.0,  FN = 2.89,  2ω = 30.1°-12.2°                     r        d           υ                                                                           n                                          ______________________________________                                        1       102.933  3.00        25.4  1.80518                                    2       72.817   10.30       82.6  1.49782                                    3       -1208.506                                                                              0.20                                                         4       107.315  7.20        82.6  1.49782                                    5       1107.838 (d5 = variable)                                              6       -427.809 3.90        35.7  1.62588                                    7       -82.554  1.60        60.1  1.62041                                    8       63.036   (d8 = variable)                                              9       -120.975 1.50        64.1  1.51680                                    10      38.555   4.50        25.5  1.80458                                    11      129.984  3.50                                                         12      -94.774  1.60        53.9  1.71300                                    13      122.725  (d13 = variable)                                             14      -496.023 4.50        60.3  1.51835                                    15      -57.366  0.20                                                         16      78.171   8.50        60.7  1.56384                                    17      -50.040  1.70        31.6  1.75692                                    18      -1381.323                                                                              (d18 = variable)                                             19      42.137   6.00        82.6  1.49782                                    20      155.005  0.22                                                         21      32.919   6.00        59.7  1.53996                                    22      41.016   1.00                                                         23      39.546   3.27        25.5  1.80458                                    24      27.620   21.95                                                        25      115.322  6.00        40.3  1.60717                                    26      -46.951  2.63                                                         27      -33.211  3.27        45.0  1.74400                                    8       -139.379 (Bf = variable)                                              ______________________________________                                        (variable spacing for vari-focusing)                                          ______________________________________                                        f       81.9953       134.9969 195.9952                                       d.sub.0 ∞       ∞  ∞                                        d.sub.5 3.1983        28.3818  40.5070                                        d.sub.8 16.6627       16.6627  16.6627                                        d.sub.13                                                                              24.5540       14.8062  -3.5877                                        d.sub.18                                                                              18.5793       3.1436   2.2369                                         Bf      59.4549       59.4548  59.4546                                        β  -0.0507       -0.0809  -0.1157                                        d.sub.0 1775.0088     1775.0089                                                                              1775.0091                                      d.sub.5 12.8409       37.6425  50.1466                                        d.sub.8 7.0201        7.4020   7.0231                                         d.sub.13                                                                              24.5540       14.8062  3.5877                                         d.sub.18                                                                              18.5793       3.1436   2.2369                                         Bf      59.4549       59.4548  59.4546                                        ______________________________________                                        (condition-corresponding numerical values)                                    ______________________________________                                        (1)           |β.sub.F |.sub.Min = 4.688               (3)           f.sub.1 (f.sub.1 + d.sub.F,R)/(d.sub.F,R × f.sub.T) =                   5.300                                                           ______________________________________                                    

From each embodiment, it is apparent that the focusing group G2F iscompact and the amount of movement of the focusing group G2F at the samephotographing distance is substantially constant irrespective of eachvari-focused position.

What is claimed is:
 1. A zoom lens comprising, in the following orderfrom the object side:a first lens group having positive refractivepower; a second lens group including a first sub-group having negativerefractive power near to said first lens group and a second sub-groupfar from said first lens group and adjacent to said first sub-group,said first lens group and said first sub-group in said second lens grouptogether forming a substantially afocal system in at least a portion ofa zooming range; a third lens group; and a fourth lens group; whereinsaid first sub-group in said second lens group is moved along theoptical axis to thereby effect focusing, said second sub-group does notmove along the optical axis during focusing, and an air gap adjacent tosaid first sub-group is varied during zooming.
 2. A zoom lens accordingto claim 1, satisfying the following condition in the whole of thezooming range:

    |β.sub.F |>3.3,

where |β_(F) | represents the absolute value of the lateralmagnification β_(F) of the first sub-group in said second lens group inan infinity state.
 3. A zoom lens according to claim 1, satisfying thefollowing condition in the whole of the zooming range:

    |β.sub.F |>1.8,

where |β_(F) | represents the absolute value of the lateralmagnification β_(F) of the first sub-group in said second lens group inan infinity state.
 4. A zoom lens according to claim 2, wherein thespacing between the first and second sub-groups is fixed during zoomingand wherein the following condition is satisfied:

    4<f.sub.1 (f.sub.1 +d.sub.F,R)/(d.sub.F,R ×f.sub.T)<15,

whered_(F),R : the spacing between the first sub-group and the secondsub-group in the second lens group during an infinity state; f₁ : thefocal length of the first lens group; f_(T) : the focal length of theentire optical system at the telephoto end.
 5. A zoom lens according toclaim 1, wherein an air gap between said first sub-group and said secondsub-group is kept constant during zooming.
 6. In a zoom lens having afirst lens portion having positive refractive power closest to an objectside and a second lens portion closest to an image side, the improvementwherein the second lens portion includes a first lens unit that isclosest to said first lens portion, that is movable along the opticalaxis to effect focusing, that has negative refractive power, that formswith said first lens portion a substantially afocal system in at least aportion of a zooming range, and that has an air gap adjacent theretothat is varied during zooming, and the second lens portion furtherincludes a second lens unit, which does not move along the optical axisduring focusing and is adjacent to said first lens unit.
 7. A zoom lensaccording to claim 6, wherein an air gap between said first lens unit insaid second lens portion and said second lens unit in said second lensportion is kept constant during zooming.
 8. A zoom lens comprising inthe following order from the object side:a first lens unit havingpositive refractive power; a focusing lens unit having negativerefractive power, and movable along the optical axis; and an imaginglens unit adjacent to said focusing lens unit; wherein said first lensunit and said focusing lens unit form a substantially afocal system inat least a portion of a zooming range, an air gap adjacent to saidfocusing lens unit and said first lens unit is varied during zooming,and said imaging lens unit does not move along the optical axis duringfocusing.
 9. A zoom lens according to claim 8, wherein an air gapbetween said focusing lens unit and said imaging lens unit is keptconstant during zooming.